The pleiotropic effects of estrogens in mammalian tissues have been well documented, and it is now appreciated that estrogens affect many organ systems [Mendelsohn and Karas, New England Journal of Medicine 340: 1801–1811 (1999), Epperson, et al., Psychosomatic Medicine 61: 676–697 (1999), Crandall, Journal of Womens Health & Gender Based Medicine 8: 1155–1166 (1999), Monk and Brodaty, Dementia & Geriatric Cognitive Disorders 11: 1–10 (2000), Hurn and Macrae, Journal of Cerebral Blood Flow & Metabolism 20: 631–652 (2000), Calvin, Maturitas 34: 195–210 (2000), Finking, et al., Zeitschrift fur Kardiologie 89: 442–453 (2000), Brincat, Maturitas 35: 107–117 (2000), Al-Azzawi, Postgraduate Medical Journal 77: 292–304 (2001)]. Estrogens can exert effects on tissues in several ways. Probably, the most well characterized mechanism of action is their interaction with estrogen receptors leading to alterations in gene transcription. Estrogen receptors are ligand-activated transcription factors and belong to the nuclear hormone receptor superfamily. Other members of this family include the progesterone, androgen, glucocorticoid and mineralocorticoid receptors. Upon binding ligand, these receptors dimerize and can activate gene transcription either by directly binding to specific sequences on DNA (known as response elements) or by interacting with other transcription factors (such as AP1), which in turn bind directly to specific DNA sequences [Moggs and Orphanides, EMBO Reports 2: 775–781 (2001), Hall, et al., Journal of Biological Chemistry 276: 36869–36872 (2001), McDonnell, Principles Of Molecular Regulation. p351–361 (2000)]. A class of “coregulatory” proteins can also interact with the ligand-bound receptor and further modulate its transcriptional activity [McKenna, et al., Endocrine Reviews 20: 321–344 (1999)]. It has also been shown that estrogen receptors can suppress NFκB-mediated transcription in both a ligand-dependent and independent manner [Quaedackers, et al., Endocrinology 142: 1156–1166 (2001), Bhat, et al., Journal of Steroid Biochemistry & Molecular Biology 67: 233–240 (1998), Pelzer, et al., Biochemical & Biophysical Research Communications 286: 1153–7 (2001)].
Estrogen receptors can also be activated by phosphorylation. This phosphorylation is mediated by growth factors such as EGF and causes changes in gene transcription in the absence of ligand [Moggs and Orphanides, EMBO Reports 2: 775–781 (2001), Hall, et al., Journal of Biological Chemistry 276: 36869–36872 (2001)].
A less well-characterized means by which estrogens can affect cells is through a so-called membrane receptor. The existence of such a receptor is controversial, but it has been well documented that estrogens can elicit very rapid non-genomic responses from cells. The molecular entity responsible for transducing these effects has not been definitively isolated, but there is evidence to suggest it is at least related to the nuclear forms of the estrogen receptors [Levin, Journal of Applied Physiology 91: 1860–1867 (2001), Levin, Trends in Endocrinology & Metabolism 10: 374–377 (1999)].
Two estrogen receptors have been discovered to date. The first estrogen receptor was cloned about 15 years ago and is now referred to as ERα [Green, et al., Nature 320: 134–9 (1986)]. The second was found comparatively recently and is called ERβ [Kuiper, et al., Proceedings of the National Academy of Sciences of the United States of America 93: 5925–5930 (1996)]. Early work on ERβ focused on defining its affinity for a variety of ligands and, indeed, some differences with ERα were seen. The tissue distribution of ERβ has been well mapped in the rodent and it is not coincident with ERα. Tissues such as the mouse and rat uterus express predominantly ERα, whereas the mouse and rat lung express predominantly ERβ [Couse, et al., Endocrinology 138: 4613–4621 (1997), Kuiper, et al., Endocrinology 138: 863–870 (1997)]. Even within the same organ, the distribution of ERα and ERβ can be compartmentalized. For example, in the mouse ovary, ERβ is highly expressed in the granulosa cells and ERα is restricted to the thecal and stromal cells [Sar and Welsch, Endocrinology 140: 963–971 (1999), Fitzpatrick, et al., Endocrinology 140: 2581–2591 (1999)]. However, there are examples where the receptors are coexpressed and there is evidence from in vitro studies that ERα and ERβ can form heterodimers [Cowley, et al., Journal of Biological Chemistry 272: 19858–19862 (1997)].
The most potent endogenous estrogen is 17β-estradiol. A large number of compounds have been described that either mimic or block the activity of 17β-estradiol. Compounds having roughly the same biological effects as 17β-estradiol are referred to as “estrogen receptor agonists”. Those which block the effect of 17β-estradiol, when given in combination with it, are called “estrogen receptor antagonists”. In reality, there is a continuum between estrogen receptor agonist and estrogen receptor antagonist activity and some compounds behave as estrogen receptor agonists in some tissues but estrogen receptor antagonists in others. These compounds with mixed activity are called selective estrogen receptor modulators (SERMS) and are therapeutically useful agents (e.g. EVISTA) [McDonnell, Journal of the Society for Gynecologic Investigation 7: S10–S15 (2000), Goldstein, et al., Hurnan Reproduction Update 6: 212–224 (2000)]. The precise reason why the same compound can have cell-specific effects has not been elucidated, but the differences in receptor conformation and/or in the milieu of coregulatory proteins have been suggested.
It has been known for some time that estrogen receptors adopt different conformations when binding ligands. However, the consequence and subtlety of these changes only recently has been revealed. The three dimensional structures of ERα and ERβ have been solved by co-crystallization with various ligands and clearly show the repositioning of helix 12 in the presence of an estrogen receptor antagonist, which sterically hinders the protein sequences required for receptor-coregulatory protein interaction [Pike, et al., Embo 18: 4608–4618 (1999), Shiau, et al., Cell 95: 927–937 (1998)]. In addition, the technique of phage display has been used to identify peptides that interact with estrogen receptors in the presence of different ligands [Paige, et al., Proceedings of the National Academy of Sciences of the United States of America 96: 3999–4004 (1999)]. For example, a peptide was identified that distinguished between ERα bound to the full estrogen receptor agonists 17β-estradiol and diethylstilbesterol. A different peptide was shown to distinguish between clomiphene bound to ERα and ERβ. These data indicate that each ligand potentially places the receptor in a unique and unpredictable conformation that is likely to have distinct biological activities.
As mentioned above, estrogens affect a panoply of biological processes. In addition, where gender differences have been described (e.g. disease frequencies, responses to challenge, etc), it is possible that the explanation involves the difference in estrogen levels between males and females.